Prefabricated stress relief cone



A. CONANGLA 2,945,913

FREFABRICATED STRESS RELIEF CONE Filed April 27, 1956 July 19, 1960 EQUIPOTENTIAL SURFACES x-INSULATION GROUND SHIELD-7 CONDUCTO R 7 EQUIPOTENTIALI SURFACES CONDUCTIVE MATERIAL SURFACE ORIGINAL GROUND SHIELD XINSULATION lCONDUCTOR ADDITIONAL INSULATION EMBEDDED CONDUCTIVE MATERIAL' GROUND CLAMP CABLE GROUND SHIELD CAST DR MOULDED HOMOGENEOUS msuumou ORIGINAL FILLER INSULATION CONDUCTOR IN VEN TOR. AMADO CONANGLA ATTORNEY Unied Sees Patent l nnnlsiinichrni) coNE "eastern, Philadelphia,- Pa, s'sinar to 'I-T-E Circuit"Breaker companfilhila'dlphia, Pa, a corporation of Pennsylvania I Filed Apr. 27, No.

2 Claims. (Cl. 17473) lMyl invjention relates to a prefabricated stress relief cone for preventing electrical breakdown at the terminal p i s. f high-vol g g und-,sh sds s High voltage cables are normally provided Wlthj ground Zshields .for. the purpose of controlling the dielectric stresses onthe cable insulation, The groundshield con- .sists: of :a thinlayer of conducting materiaLimmediate- -ly adjacent to. the. outermost layer ofinsulation on the cable, and connectedv to-ground. This ground shield constitutesjin'eifect, an .equipotential surface concentric with the axis of the conductor, and. its presence causes the electric field distribution around 'the conductor to be radial, -with-maximum potential gradient at every point predictable by calculation, with zero 'gradients'in "the longitudinal direction ofithe cable.

"At the cable termination, the ground shield must be removed s'ufiicieritlylfar back from the exposed conductor 1' end i to p event electrical breakdown of the'adjacent air between T thef exposed co'nductorand theground shield *eii'dtepr vem"crepa e along the surface of the cable setwee the" in excessive potential points. This, however, results V A die'n'ts near the terminal edge e ground shield 'as hereinafter illustrated, in more detail In :tp "prevent electrical T break down at cable termination points, it' has been customary at'the termination or the ground shield to'p i'ovide additional insulation and a generally bell 'shaped 'extension to the ground shield Thisass embly', well known tetheart, "is calleda dielectric 'str'eiss: relief cone, Tor more b'rieflyfa stress cone.

Such stress cones, howeverfare completely dependent on the skillofithe workman who winds the tape and often are uhpredi'c'tablefin their dielectric qualities be cause of air spaces, poor centering, and lack of homogeneity .of the insulating, material.

Thdpres'ntinvention overcomes these deficiencies by prOvidingQa prefabricated stress reliefpone made of a eastr'esimwhichcan be rnoldedto provide a unit of predetermined fand, consistent dielectric strength. Such a zunit,is,. also readily installed byeven' relativelv'unskilled workmen...

Accordingly, Zitiiis E object of a "invention to pro.

v ide a prefabricatedstress relief cone which may be accuratly produced. and centered .v h 9 l 8- cable axis independentlyoft the skill of the workmen attaching the s ame to the'cable,

Q Another object off'my'"inviintionis "to" provide a 'stress reliefconewliich can be made i in any predetermined shape and of predetermined dielectric strength so as to provide consistently and ac'cu'rately the"proper degree of insulation at cable teiirninationpoints. 7

Still another object of my invention is to provide a stress cone which is easily installed by relatively unskilled workmen.

These and other objects of my invention will become 2,945,913 Patented July "19, 1-9 0 ice apparent froin the following description when taken in connection with the drawingsrin which:

, Figure l is illustrative ofthe field arouud a conductor showing the chauge of such field at the'point of termination of a ground shield. ,7 g g I,

Figure 2 shows the electricfield as it would be distributed when a tape wound stress 7 relief cone of the prior art is applied at the termination point of the ground shield. V

Figure 3 illustrates the cast stress relief cone of the presentinvention as it would appear when assembled around the cable.

Figure 4 'is another embodiment of the present invention shown in Figure 3 in which the external surface of the insulating material is corrugated so as to increase resistance to cree'page.

Essentially the present invention comprises a prefabricated stress relief cone for preventing electrical breakdown at cable termination points, which cone comprises a mass of homogeneous insulating material having a cylindrical hole therethrough, a conductive materialembedded in said insulating material, and means for electrically securing said conductive material to't he cable "ground shield.

Referring first to'Fig ure 1, it"ca'n be seen that the electric field around the conductor is normally radial.

Therefore, at the cable termination, the ground "shield must be removed sufiiciently far back from the exposed conductor end'to'prje'vent electric breakdown'of the 'adjacent air between thejexposed conductbr andthe ground shield, or creepage along the surface of ithe cable insulation between the'same points. The removal of the gr'oun d'shield for any' specified length from the termination is not'a sufiicient condition for preventing dielectric failure from conductor to ground, for the reason that In order to pievent cable insulation failure, it has been customary to provide-at the termination of the 'as'known to the prior art.

'[asto obtain the desired cross sectional contour. ductingltape,' foil or gauze is then'placed over the addifi 'j'tional insulation, and electrically bonded-to the cable ground shield a generally bell-shaped extension "of'the type shown in FigureZ. This'is the typical stress come I Here the additional insulation is built up by winding layers ofv insulating tape so Con- "ground shield. 'A' sufficient lengthof the original'c'able insulation is'left between the cable end and thestress gent extension of the ground -shield on I the cable.

cone, to extend the creepage path from conductor to groundshield over'the surface of this insulation.

The bell-shaped conductive layer constitutes a diver- Its presence serves to control the configuration of the elect'riefi'eld both radiallyand longitudinally so that the J' 'p oten'tial gradients are ke'ptwithin "safe limits. For best "results, the stress cone profile should follow a curvecorresponding to a mathematically derivedfunctiom'and the additional insulation should contain no air spaces where corona can occur. These requirements are very diflicultto satisfy when the stress cone is built up in the 3 231 of wound layers of tape, and require considerable s In accordance with the present invention, a prefabrica-ted stress relief cone is provided which can be installed in the field by a relatively unskilled workman. As seen in Figure 3, a conductive material which is of a generally conical shape and diverges from the axis of the cable is embedded in a mass of cast homogeneous insulating material. This mass of insulating material has a cylindrical hole therethrough, concentric with the axis of the conical shaped conductive layer, and having a diameter slightly larger than the outside diameter of the cable insulation.

The narrow end of the embedded conductive layer is bonded electrically to a cylindrical conductive piece or ground clamp that extends towards the rear, thereby electrically bonding the conductive material to the cable ground shield. The external surface can be plain, as shown in Figure 3, or corrugated as shown in Figure 4 to increase the surface creepage distance in the longitudinal direction. The air space between the internal wall of the cylindrical hole and the outer surface of the cable surface is filled at the time of installation with a resin curable at room temperature.

The insulating material comprises a cast resin having high dielectric properties. Such resins would include various rubbers, natural and synthetic, halogenated rubbers, polyester and epoxy resins. Of the above, however, because of its excellent casting mechanical and electrical properties, I prefer to use the epoxy resins.

The epoxy resins herein referred to are of the type comprising the resinous product of reaction between an epihalohydrin such as epichlorohydrin and a polyhydroxyphenol such as bis-phenol-A. Resins of this type are referred to in US. Patents Nos. 2,324,483, 2,444,333 and 2,458,796. These resins have been found to possess excellent mechanical resistance, resistance to water and alkali and outstanding electrical insulation properties.

. They are sold under such trade names as Araldite and "fEpon. Depending upon the desired physical properties, various fillers such as slate, quartz, flint and clays may be incorporated with the epoxy resins.

A sample formulation of the resin used in connection with my invention is as follows:

100 parts, by weight, Araldite 6060 epoxy resin (This is a 100% solids content epoxy resin of the acid catalysis type, 'i.e. an epoxy resin in which the hard infusible product is obtained through the addition of a polycarboxyli'c acid type cross-linking agent. See United States Patent No. 2,712,535.)

200 parts, by weight, of silica filler (99.8% pure) 30 parts, by weight, of phthalic anhydride I The above mixture is heated to 300 F. and poured under vacuum into a mold. The resin is cured at 320 F. for one hour at atmospheric pressure.

The advantages of the present invention are many. First, the prefabricated stress cone .is easily and simply installed. The installation steps would consist merely of stripping the ground shield for the required length, slipping the stress cone on the cable and back to the ground shield termination, electrically bonding the stress cone to the cable ground shield, and filling the air space with the filler or liquid insulating compound curable at room temperature.

The conductive surface constituting the field control ling element can be accurately produced and centered on the cable axis independently of the skill'of a workman. This surface can be made to follow any mathematically derived curve without the compromises required when the surface is built up by'means of successive layers of tape.

Moreover, air spaces where corona could occur are posi- V tively eliminated, 1

The use of homogeneous insulating material insures equal dielectric strength in all directions and permits reducing the overall dimensions of the cone. In the previously used cone, the dielectric strength along the tape layers is much smaller than across them.

The total length of original cable insulation necessary beyond the stress cone for surface creepage considerations can be reduced due to the additional creepage distance provided by this device. A further reduction in the overall length ofthe cable termination is obtained when providing a corrugated surface for the cast resin surface.

Although I have here described preferred embodiments of my invention, many variations and modifications will now be apparent to those skilled in the art. I prefer to be limited, therefore, not by the specific disclosure herein, but only by the appended claims.

I claim:

1. A prefabricated stress relief cone for the terminal end of a high voltage ground shielded cable to prevent electrical breakdown of said cable; said ground shielded cable comprising a central conductor, a tube of solid insulation surounding said central-conductor and a ground shield of conductive material surrounding said tube of solid insulation; said stress relief cone consisting of a sheath of conductive material and an epoxy resin; said epoxy resin being molded around said conductive material and having a cylindrical opening along its axis; said sheath of conductive material having substantially smooth surfaces for its entire length; said sheath of conductive material having a generally conical shape which diverges along the complete length thereof from the axis of the cone; said epoxy resin forming a mass of homogeneous insulating material and encapsulating said conductive sheath; said conductive sheath having connecting means extending therefrom to electrically connect said conductive sheath to said ground shield; said connecting means comprising an extending portion of said cone at the small diameter thereof; said terminal end of said high voltage cable having said ground shield removed therefrom to expose the outer surface of said tube of solid insulation; said prefabricated stress relief cone being removably connected to said terminal end of said high voltage cable and receiving said exposed surface of said solid insulation in said cylindrical opening of said stress relief cone; said extending portion of said cone having an inner diameter substantially equal to the inner diameter of said ground shield; the end of said extending portion of said cone being adjacent the end of said ground shield.

2. The device as defined in claim 1 wherein the external surface of said epoxy resin mass is corrugated to increase the creepage distance along said surface.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Article I, Ethoxylines (Preiswerk et al.), published in Modern Plastics, November 1950 (pages -88 relied on). I

Davidson: Designing Potted Circuits, Electronics Deg March 1955, pages 38 and 39, 

